Re: [RFC PATCH bpf-next 1/2] bpf, x64: Fix tailcall infinite loop bug

[Leon Hwang]

On 15/8/23 08:52, Eduard Zingerman wrote:
> On Mon, 2023-08-14 at 21:41 +0800, Leon Hwang wrote:
>> From commit ebf7d1f508a73871 ("bpf, x64: rework pro/epilogue and tailcall
>> handling in JIT"), the tailcall on x64 works better than before.
>>
>> From commit e411901c0b775a3a ("bpf: allow for tailcalls in BPF subprograms
>> for x64 JIT"), tailcall is able to run in BPF subprograms on x64.
>>
>> From commit 5b92a28aae4dd0f8 ("bpf: Support attaching tracing BPF program
>> to other BPF programs"), BPF program is able to trace other BPF programs.
>>
>> How about combining them all together?
>>
>> 1. FENTRY/FEXIT on a BPF subprogram.
>> 2. A tailcall runs in the BPF subprogram.
>> 3. The tailcall calls itself.
>>
>> As a result, a tailcall infinite loop comes up. And the loop halts the
>> machine.
>>
>> As we know, in tail call context, the tail_call_cnt propagates by stack
>> and RAX register between BPF subprograms. So do it in FENTRY/FEXIT
>> trampolines.
> 
> Hi Leon,
> 
> I'm not familiar with this part of the jit compiler, so decided that
> taking a look at your series might be a good learning point.
> I think I got the gist of it, but I don't understand where
> the initial value of RAX (== 0) is coming from in
> arch_prepare_bpf_trampoline(), could you please help me out?
> 
> Also a nitpick:
> - in arch_prepare_bpf_trampoline() there is a comment detailing 
>   the stack layout, it probably should be updated to say that
>   tail call count is stored as well;
> - before arch_prepare_bpf_trampoline() there is a comment with
>   an example of generated assembly, should it be updated?
> 
> Thanks,
> Eduard
> 

a) Initial value of RAX is in emit_prologue().
	if (!ebpf_from_cbpf) {
		if (tail_call_reachable && !is_subprog)
			/* When it's the entry of the whole
			 * tailcall context, zeroing the RAX
			 * means init tail_call_cnt.
			 */
			EMIT2(0x31, 0xC0); /* xor eax, eax */
		else
			// Keep the same asm layout.
			EMIT2(0x66, 0x90); /* nop2 */
	}
   I'd like to add this comment to emit_prologue().

b) Good to update the stack layout. I'll do it.

c) Its comment will be updated also.

Thanks,
Leon

>>
>> Fixes: ebf7d1f508a7 ("bpf, x64: rework pro/epilogue and tailcall handling in JIT")
>> Fixes: e411901c0b77 ("bpf: allow for tailcalls in BPF subprograms for x64 JIT")
>> Signed-off-by: Leon Hwang <hffilwlqm@xxxxxxxxx>
>> ---
>>  arch/x86/net/bpf_jit_comp.c | 23 +++++++++++++++++++----
>>  include/linux/bpf.h         |  6 ++++++
>>  kernel/bpf/trampoline.c     |  5 +++--
>>  kernel/bpf/verifier.c       |  9 +++++++--
>>  4 files changed, 35 insertions(+), 8 deletions(-)
>>
>> diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
>> index a5930042139d3..ca5366d97ad04 100644
>> --- a/arch/x86/net/bpf_jit_comp.c
>> +++ b/arch/x86/net/bpf_jit_comp.c
>> @@ -1018,6 +1018,10 @@ static void emit_shiftx(u8 **pprog, u32 dst_reg, u8 src_reg, bool is64, u8 op)
>>  
>>  #define INSN_SZ_DIFF (((addrs[i] - addrs[i - 1]) - (prog - temp)))
>>  
>> +/* mov rax, qword ptr [rbp - rounded_stack_depth - 8] */
>> +#define RESTORE_TAIL_CALL_CNT(stack)				\
>> +	EMIT3_off32(0x48, 0x8B, 0x85, -round_up(stack, 8) - 8)
>> +
>>  static int do_jit(struct bpf_prog *bpf_prog, int *addrs, u8 *image, u8 *rw_image,
>>  		  int oldproglen, struct jit_context *ctx, bool jmp_padding)
>>  {
>> @@ -1623,9 +1627,7 @@ st:			if (is_imm8(insn->off))
>>  
>>  			func = (u8 *) __bpf_call_base + imm32;
>>  			if (tail_call_reachable) {
>> -				/* mov rax, qword ptr [rbp - rounded_stack_depth - 8] */
>> -				EMIT3_off32(0x48, 0x8B, 0x85,
>> -					    -round_up(bpf_prog->aux->stack_depth, 8) - 8);
>> +				RESTORE_TAIL_CALL_CNT(bpf_prog->aux->stack_depth);
>>  				if (!imm32)
>>  					return -EINVAL;
>>  				offs = 7 + x86_call_depth_emit_accounting(&prog, func);
>> @@ -2464,6 +2466,8 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
>>  	else
>>  		/* sub rsp, stack_size */
>>  		EMIT4(0x48, 0x83, 0xEC, stack_size);
>> +	if (flags & BPF_TRAMP_F_TAIL_CALL_CTX)
>> +		EMIT1(0x50);		/* push rax */
>>  	/* mov QWORD PTR [rbp - rbx_off], rbx */
>>  	emit_stx(&prog, BPF_DW, BPF_REG_FP, BPF_REG_6, -rbx_off);
>>  
>> @@ -2516,6 +2520,12 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
>>  		restore_regs(m, &prog, regs_off);
>>  		save_args(m, &prog, arg_stack_off, true);
>>  
>> +		if (flags & BPF_TRAMP_F_TAIL_CALL_CTX)
>> +			/* Before calling the original function, restore the
>> +			 * tail_call_cnt from stack.
>> +			 */
>> +			RESTORE_TAIL_CALL_CNT(stack_size);
>> +
>>  		if (flags & BPF_TRAMP_F_ORIG_STACK) {
>>  			emit_ldx(&prog, BPF_DW, BPF_REG_0, BPF_REG_FP, 8);
>>  			EMIT2(0xff, 0xd0); /* call *rax */
>> @@ -2569,7 +2579,12 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
>>  			ret = -EINVAL;
>>  			goto cleanup;
>>  		}
>> -	}
>> +	} else if (flags & BPF_TRAMP_F_TAIL_CALL_CTX)
>> +		/* Before running the original function, restore the
>> +		 * tail_call_cnt from stack.
>> +		 */
>> +		RESTORE_TAIL_CALL_CNT(stack_size);
>> +
>>  	/* restore return value of orig_call or fentry prog back into RAX */
>>  	if (save_ret)
>>  		emit_ldx(&prog, BPF_DW, BPF_REG_0, BPF_REG_FP, -8);
>> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
>> index cfabbcf47bdb8..55c72086034ef 100644
>> --- a/include/linux/bpf.h
>> +++ b/include/linux/bpf.h
>> @@ -1028,6 +1028,11 @@ struct btf_func_model {
>>   */
>>  #define BPF_TRAMP_F_SHARE_IPMODIFY	BIT(6)
>>  
>> +/* Indicate that current trampoline is in a tail call context. Then, it has to
>> + * cache and restore tail_call_cnt to avoid infinite tail call loop.
>> + */
>> +#define BPF_TRAMP_F_TAIL_CALL_CTX	BIT(7)
>> +
>>  /* Each call __bpf_prog_enter + call bpf_func + call __bpf_prog_exit is ~50
>>   * bytes on x86.
>>   */
>> @@ -1147,6 +1152,7 @@ struct bpf_attach_target_info {
>>  	struct module *tgt_mod;
>>  	const char *tgt_name;
>>  	const struct btf_type *tgt_type;
>> +	bool tail_call_ctx;
>>  };
>>  
>>  #define BPF_DISPATCHER_MAX 48 /* Fits in 2048B */
>> diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
>> index 78acf28d48732..0fae334e3f7b8 100644
>> --- a/kernel/bpf/trampoline.c
>> +++ b/kernel/bpf/trampoline.c
>> @@ -415,8 +415,8 @@ static int bpf_trampoline_update(struct bpf_trampoline *tr, bool lock_direct_mut
>>  		goto out;
>>  	}
>>  
>> -	/* clear all bits except SHARE_IPMODIFY */
>> -	tr->flags &= BPF_TRAMP_F_SHARE_IPMODIFY;
>> +	/* clear all bits except SHARE_IPMODIFY and TAIL_CALL_CTX */
>> +	tr->flags &= (BPF_TRAMP_F_SHARE_IPMODIFY | BPF_TRAMP_F_TAIL_CALL_CTX);
>>  
>>  	if (tlinks[BPF_TRAMP_FEXIT].nr_links ||
>>  	    tlinks[BPF_TRAMP_MODIFY_RETURN].nr_links) {
>> @@ -783,6 +783,7 @@ struct bpf_trampoline *bpf_trampoline_get(u64 key,
>>  
>>  	memcpy(&tr->func.model, &tgt_info->fmodel, sizeof(tgt_info->fmodel));
>>  	tr->func.addr = (void *)tgt_info->tgt_addr;
>> +	tr->flags = (tgt_info->tail_call_ctx ? BPF_TRAMP_F_TAIL_CALL_CTX : 0);
>>  out:
>>  	mutex_unlock(&tr->mutex);
>>  	return tr;
>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
>> index 4ccca1f6c9981..a78e5a2ae5c72 100644
>> --- a/kernel/bpf/verifier.c
>> +++ b/kernel/bpf/verifier.c
>> @@ -19400,10 +19400,15 @@ int bpf_check_attach_target(struct bpf_verifier_log *log,
>>  			return -EINVAL;
>>  		fallthrough;
>>  	case BPF_MODIFY_RETURN:
>> -	case BPF_LSM_MAC:
>> -	case BPF_LSM_CGROUP:
>>  	case BPF_TRACE_FENTRY:
>>  	case BPF_TRACE_FEXIT:
>> +		if (tgt_prog && subprog > 0 &&
>> +		    tgt_prog->aux->func[subprog]->is_func &&
>> +		    tgt_prog->aux->tail_call_reachable)
>> +			tgt_info->tail_call_ctx = true;
>> +		fallthrough;
>> +	case BPF_LSM_MAC:
>> +	case BPF_LSM_CGROUP:
>>  		if (!btf_type_is_func(t)) {
>>  			bpf_log(log, "attach_btf_id %u is not a function\n",
>>  				btf_id);
>